Gas sensor for determining the concentration of gas components in gas mixtures and use thereof

ABSTRACT

A gas sensor for determining the concentration of gas components in gas mixtures is used in particular to measure the concentration of nitrogen oxides in exhaust gases of internal combustion engines or in the interiors of motor vehicles. It contains an electrochemical measuring cell which includes a first electrode ( 31 ) situated on a solid electrolyte and an additional electrode ( 32 ). The electrode ( 31 ) is made of an oxide material containing lanthanum which at least catalytically decomposes nitrogen oxides. The pump current flowing between the electrodes ( 31, 32 ) is used as a measure of the nitrogen oxide concentration in the gas mixture.

[0001] The invention relates to a gas sensor for determining gas components according to the preamble of the main claim.

BACKGROUND INFORMATION

[0002] A sensor element for determining the NO_(x) concentration in gas mixtures is known from German Published Application 196 52 968 which is based on the interaction of two electrochemical pump cells. Two internal pump electrodes that are interconnected with a common external pump electrode arranged in a reference gas channel are located in a measuring gas space of the sensor element. The first of the two pump cells in the direction of inflow of the gas mixture causes oxygen to be transported from the measuring gas space into the reference gas channel. The internal pump electrode of this first oxygen-transporting pump cell is covered with a multilayered structure made of a mixed-conducting metal oxide layer and an electrically insulating aluminum oxide layer, which selectively removes the oxygen present in the gas mixture without changing the concentration of nitrogen oxides. These are then decomposed at the internal pump electrode of the second pump cell and the oxygen released in doing so is pumped off. The pump current of the second pump cell is used as a measure of the concentration of nitrogen oxides contained in the gas mixture.

[0003] A gas sensor for determining the NO_(x) concentration in gas mixtures is also known from European Patent Application 678 740 A1. It includes two measuring gas spaces with one pump cell each, which are arranged adjacent to each other in a layer plane of a planar ceramic substrate. The measuring gas flows through a diffusion opening into the first measuring gas space in which a first pump cell is located. The first pump cell is used in the first measuring gas space to set a predetermined oxygen partial pressure by pumping oxygen in or out. A concentration cell also situated in the first measuring gas space makes it possible to maintain a constantly low oxygen partial pressure in the first measuring gas space by determining the electrical voltage (electromotive force) present at the electrodes of the concentration cell. Via an additional diffusion opening, the gas mixture set to a constant oxygen partial pressure enters the second measuring space. An additional pump cell is situated in the second measuring gas space. Its internal pump electrode is made of rhodium and it makes it possible to decompose nitrogen oxides to N₂ and O₂. The reduced oxygen arising at the internal pump electrode is pumped off via an applied pump voltage. The pump current of the second pump cell is proportional to the nitrogen oxide concentration of the gas mixture.

[0004] In both cases, a constantly low oxygen partial pressure of the gas mixture in the sensor element must be set in an elaborate manner before the nitrogen oxides can be determined with the internal pump electrodes used in them.

ADVANTAGES OF THE INVENTION

[0005] The gas sensor according to the present invention having the characterizing features of the independent claims has the advantage that an electrochemical measuring cell, the NO_(x)-sensitive electrode of which is made of a material that makes it possible to determine the nitrogen oxide concentration in a gas mixture reliably even at high oxygen partial pressures, is used to determine the nitrogen oxide concentration in the measuring gas. This makes it unnecessary to install oxygen-transporting pump cells into the sensor element and therefore considerably simplifies the sensor design.

[0006] The measures cited in the dependent claims make advantageous refinements and improvements of the gas sensor specified in the main claim possible. Thus, for example, the use of the NO_(x)-sensitive pump cell according to the invention makes it possible to omit the incorporation of a measuring gas space and a reference gas channel into the sensor element on which the gas sensor is based since the NO_(x)-sensitive pump electrode as well as the counter-electrode may be directly exposed to the exhaust gas. Of particular advantage is a sandwich arrangement of both electrodes one above the other on the wide surface area of the sensor element.

[0007] However, if a reference gas channel is provided in the sensor element, a reference electrode situated there together with the NO_(x)-sensitive electrode makes it possible to determine the nitrogen dioxide concentration of the gas mixture simultaneously using a voltage measurement as an alternative.

DRAWING

[0008] An exemplary embodiment of the invention is shown in the drawing and explained in greater detail in the following description.

[0009]FIG. 1 shows a cross section through the wide surface area of a sensor element on which the gas sensor according to the invention is based and

[0010]FIGS. 2 and 3 show a sensor element according to two additional exemplary embodiments.

EXEMPLARY EMBODIMENTS

[0011]FIG. 1 shows a basic structure of a first embodiment of a planar sensor element 10 of an electrochemical gas sensor. Sensor element 10 has, for example, a plurality of oxygen ion-conducting solid electrolyte layers 11 a, 11 b, 11 c, 11 d, 11 e and 11 f. Solid electrolyte layers 11 a-11 f are designed as ceramic films and form a planar ceramic body. The integrated form of the planar ceramic body of sensor element 10 is produced by laminating together the ceramic films printed with functional layers and subsequently sintering the laminated structure in a manner known per se. Each of solid electrolyte layers 11 a-11 f is made of oxygen ion-conducting solid electrolyte material such as, for example, ZrO₂ partially or fully stabilized with Y₂O₃.

[0012] Sensor element 10 contains a measuring gas space 13 and, for example, a reference gas channel 19 in an additional layer plane 11 d, one end of the reference gas channel leading out from the planar body of sensor element 10 and being in contact with an atmosphere of air.

[0013] Moreover, a resistance heater 40 is embedded in the ceramic body of sensor element 10 between two electrical insulation layers which are not shown here. The resistance heater is used to heat sensor element 10 to the required operating temperature.

[0014] In addition, sensor element 10 has a gas inlet opening 21 which conducts the measuring gas into first measuring space 13. Gas inlet opening 21 is, for example, situated in the same layer as measuring gas space 13. A first diffusion barrier 23 of, for example, porous ceramic material is formed at the inlet to first measuring gas space 13 downstream of gas inlet opening 21 in the direction of diffusion of the measuring gas.

[0015] An internal NO_(x)-sensitive electrode 31 is situated in measuring gas space 13. Associated external electrode 32 is located in reference gas channel 19. Both electrodes 31, 32 are interconnected with a pump cell. Electrode 32 is made of a catalytically active material, for example, platinum. The electrode material for electrode 32 is used as a cermet in a manner known per se in order to sinter it to the ceramic films. Electrodes 31, 32 are contacted via printed conductors (not shown here), which are guided between solid electrolyte layers 11 a and 11 b and are connected to the wide surface area of the sensor element via throughplating (also not shown here).

[0016] In order to ensure that the nitrogen oxides contained in the gas mixture are completely decomposed into nitrogen and oxygen on NO_(x)-sensitive electrode 31, NO_(x)-sensitive electrode 31 is made of a catalytically active, oxide material, for example, from a lanthanum-containing. perovskite of the composition La_(1−x) Sr_(x) CO_(1−y) Cu_(y) O_(3−δ). Traditionally, electrodes of this type are produced from rhodium or a platinum-rhodium alloy. The latter only allow a reliable determination of the concentration of nitrogen oxides at very low oxygen concentrations of, for example, 0.02 ppm in the gas mixture and therefore are usable only in sensors that remove the greatest proportion of the oxygen contained in the gas mixture electrochemically (see European Patent Application 678 740 A1).

[0017] The essential advantage of the electrode material of the present invention, which includes a lanthanum-containing perovskite, is that it permits a determination of the concentration of nitrogen oxides even at a 2 to 20% concentration of oxygen in the gas mixture. Although the nitrogen oxides are present in these oxygen-rich gas mixtures at an unfavorable ratio compared to oxygen of 1:1000 to 1:10000, a linear dependence of the pump current flowing in the pump cell on the nitrogen oxide concentration can be observed when lanthanum-containing perovskite is used. The oxygen present in the gas mixture is observed only in the form of a slightly elevated baseline which is hardly subject to change even when the oxygen concentration varies greatly.

[0018] This characteristic is all the more unexpected since previously it was only known that a perovskite of the same composition is able to be used as an oxygen-selective protective layer for pump cells for the removal of molecular oxygen from gas mixtures (see German Published Application 196 52 968 A1). Only molecular oxygen is absorbed on this protective layer while it is impossible to catalytically decompose nitrogen oxides. However, in contrast to the present invention, the protective layer is present as an electrically insulating metallic oxide layer and no pump voltage is applied to it.

[0019] The high measuring accuracy of the lanthanum-containing perovskite used according to the present invention as an NO_(x)-sensitive electrode 31 even at high oxygen concentrations in the gas mixture makes it possible as an alternative to arrange this electrode on the wide surface area of sensor element 10 directly exposed to the gas mixture and thus eliminate the incorporation of a measuring gas space 13 in the sensor element. If, for example, external electrode 32 is also formed on the wide surface area of the sensor element exposed to the gas mixture, the sensor design is further simplified since a reference gas channel may be omitted also. A sensor element of this design is shown in FIG. 2. To protect against contamination, electrodes 31, 32 are additionally provided with a porous gas-permeable protective layer 35 made of CeO₂, for example.

[0020] A further embodiment of the invention provides that electrodes 31, 32 are not situated adjacent to each other on the wide surface area of the sensor element, as shown in FIG. 2, but rather on top of each other as in a sandwich and separated by a porous, gas-permeable and oxygen ion-conducting solid electrolyte layer 37. An embodiment of this type is shown in FIG. 3. The arrangement of a measuring and a reference electrode in superimposed layers on the wide surface area of a sensor element is also customary in mixed potential sensors.

[0021] If the incorporation of a reference gas channel in the sensor element is not omitted, a reference electrode 33 situated in it according to the exemplary embodiment illustrated in FIG. 3 may, for example, be interconnected with NO_(x)-sensitive electrode 31 to form a concentration cell. In a particularly advantageous manner, this allows the simultaneous determination of the nitrogen oxide concentration by an amperometric method using the pump cell consisting of electrodes 31 and 32 and by a potentiometric method using the potential difference formed between electrodes 31 and 33.

[0022] It must be expressly noted that the use of a lanthanum-containing perovskite is not limited to the explained exemplary embodiments but rather this material may also be used in conventional nitrogen oxide sensors having one or more measuring gas spaces and one or more pump cells and concentration cells.

[0023] As a result of the good oxygen tolerance of the sensor element of the present invention even at atmospheric oxygen concentrations, it is also conceivable to use the sensor element in air quality sensors in addition to determining nitrogen oxide concentrations in exhaust gases of internal combustion engines. 

What is claimed is:
 1. A gas sensor for determining the concentration of gas components in gas mixtures, of NO_(x) in particular, having at least one electrochemical measuring cell, the measurement signal of which is used to determine the concentration of the gas component and which includes a first and at least one additional electrode applied to a solid electrolyte, at least one of the electrodes containing a material that at least catalytically decomposes nitrogen oxides, wherein the material that at least catalytically decomposes nitrogen oxides includes an oxide.
 2. The gas sensor according to claim 1, wherein the material that at least catalytically decomposes nitrogen oxides includes an oxide of at least one of the elements belonging to the lanthanoide group.
 3. The gas sensor according to claim 1 and 2, wherein the material that at least catalytically decomposes nitrogen oxides includes an oxide of lanthanum.
 4. The gas sensor according to at least one of claims 1 through 3, wherein the material that at least catalytically decomposes nitrogen oxides is an oxide of the composition La_(1−x) Sr_(x) CO_(1−y) Cu_(y) O_(3−δ).
 5. The gas sensor according to at least one of claims 1 through 4, wherein the first electrode (31) and the additional electrode (32) are interconnected as a pump cell and the measured pump current is a measure of the concentration of the nitrogen oxides present in the gas mixture.
 6. The gas sensor according to at least one of claims 1 through 5, wherein the first electrode (31) is situated in a measuring gas space (13) integrated in a sensor element (10), the measuring gas space being in contact with the gas mixture via a gas inlet (21), and the first electrode (31) contains the material that at least catalytically decomposes nitrogen oxides.
 7. The gas sensor according to at least one of claims 1 through 5, wherein the first electrode (31) is situated on a wide surface area of a sensor element (10) exposed to the gas mixture, and the first electrode (31) contains the material that at least catalytically decomposes nitrogen oxides.
 8. The gas sensor according to claim 7, wherein the additional electrode (32) is also situated on the wide surface area of the sensor element exposed to the gas mixture.
 9. The gas sensor according to at least one of claims 1 through 5, wherein the first electrode (31) and the additional electrode (32) are situated on a wide surface area of a sensor element (10) exposed to the gas mixture; the first electrode (31) contains the material that at least catalytically decomposes nitrogen oxides; and the sensor element has a reference gas channel (19) having a reference electrode situated (33) within it, which is connected with the first electrode (31) to form a concentration cell.
 10. The gas sensor according to claim 9, wherein a solid electrolyte layer (37) is applied to the additional electrode (32), and the first electrode (31) is applied to the solid electrolyte layer (37).
 11. Use of the gas sensor according to at least one of the preceding claims as an air quality sensor for determining the concentration of nitrogen oxides, in particular in the interior of motor vehicles.
 12. Use of the gas sensor according to at least one of the preceding claims for determining gas components, in particular NO_(x), in exhaust gases of internal combustion engines. 